In a color imaging apparatus including a single-plate color imaging element, an output image from the color imaging element is a RAW image (mosaic image). Therefore, a multi-channel image is obtained by a process of interpolating a pixel of a missing color from a surrounding pixel (demosaicing processing). In this case, there is a problem in reproduction characteristics of a high-frequency image signal.
A primary-color Bayer array as a color array most widely used in the single-plate color imaging element includes green (G) pixels arranged in a check pattern and red (R) and blue (B) arranged line-sequentially. Therefore, there is a problem of low-frequency coloring (color moire) caused by folding of high frequency signals exceeding reproduction bands of the colors and caused by deviation of phases of the colors.
A black and white vertical-striped pattern (high frequency image) as shown in FIG. 14(A) enters an imaging element in a Bayer array shown in FIG. 14(B), and the pattern is sorted into Bayer color arrays to compare the colors. As shown in FIGS. 14(C) to 14(E), R forms a light and flat color image, B forms a dark and flat color image, and G forms a light and dark mosaic color image. Although there is no density difference (level difference) between RGB with respect to the original black and white image, the image is colored depending on the color array and the input frequency.
Similarly, a black and white oblique high frequency image as shown in FIG. 15(A) enters an imaging element in a Bayer array shown in FIG. 15(B), and the image is sorted into Bayer color arrays to compare the colors. As shown in FIGS. 15(C) to 15(E), R and B form light and flat color images, while G forms a dark and flat color image. Assuming that the value of black is 0 and the value of white is 255, the black and white oblique high frequency image turns green, because only G is 255. In this way, the oblique high frequency image cannot be correctly reproduced in the Bayer array.
In the color imaging apparatus using the single-plate color imaging element, an optical low-pass filter formed by an anisotropic substance such as crystal is generally arranged on the front side of the color imaging element to prevent optically reducing the high frequency wave. However, although the coloring caused by folding of the high frequency signal can be reduced in the method, there is a problem that the resolution is reduced accordingly.
To solve the problem, a color imaging element is proposed, wherein a color filter array of the color imaging element is a three-color random array satisfying array restrictions in which an arbitrary target pixel is adjacent to three colors including the color of the target pixel on four sides of the target pixel (PTL 1).
An image sensor of a color filter array is also proposed, wherein the image sensor includes a plurality of filters with different spectral sensitivity, and first and second filters among the plurality of filters are alternately arranged in a first predetermined period in one of the diagonal directions of a pixel grid of the image sensor and are alternately arranged in a second predetermined period in the other diagonal direction (PTL 2).
Meanwhile, PTL 3 describes a technique of using surrounding pixels of a target pixel of a mosaic image in a Bayer array to calculate correlations in horizontal, vertical, and oblique (NE, NW) directions (four directions), and weights are applied according to ratios of the calculated correlations to interpolate the pixels.
A color imaging element is also proposed, wherein R and B among the three primary colors of RGB are arranged every three pixels in horizontal and vertical directions, and G is arranged between R and B (PTL 4).
A color filter array of the color imaging element includes a basic array pattern corresponding to 4×4 pixels, and the basic array pattern is repeatedly arranged in the horizontal and vertical directions. The numbers of RGB pixels in the basic array pattern are two pixels, twelve pixels, and two pixels, respectively. Therefore, the ratio of the numbers of RGB pixels is 1:6:1, and significantly more G pixels than the R and B pixels are arranged.